Question

For questions 5-9, indicate which terms (i.e. A, B, C) in the van Deemter equation will be affected and how.

5.     A smaller particle size of the stationary phase packing

A.   Decrease A only

B.    Decrease C only

C.    Increase A but decrease C

D.   Decrease both A and C

E.    Increase B

6.     A larger film thickness on the stationary phase packing

A.   Decrease B only

B.    Increase A only

C.    Increase C only

D.   Decrease C only

E.    Decrease both A and C

7.     A distribution of particle sizes in the stationary phase packing

A.   Decrease A only

B.    Increase A only

C.    Increase both A and C

D.   Decrease C but increase B

E.    Decrease A but increase C

8.     Small diffusion coefficient of the analyte

A.   Decrease B but increase C

B.    Increase C but decrease B

C.    Increase C only

D.   Increase B only

E.    Decrease A only

9.     Lower solvent viscosity (LC)

A.   Decrease B but increase C

B.    Increase C but decrease B

C.    Increase B but decrease C

D.   Increase B only

E.    Decrease C only

Answer 1

Let me explain this way -

The Van Deemter equation is an empirical formula describing the relationship between plate height (H, the length needed for one theoretical plate) which is a measure of column efficiency, and linear velocity (µ)

H=A+B/u+C×u

H = HETP (plate height)
A = eddy diffusion term
B = longitudinal diffusion term
u = linear velocity
C = Resistance to mass transfer coefficient  

So, answers are as follows -

5 = C because = The larger the particles, the stronger the dispersion effect

6 = C because  The C-term in the van Deemter equation relates to the mass transfer of sample components between the stationary phase and the mobile phase during separation.

7 = C inc both A and C. Reason is quite clear

8 =D= Increased temperature and diffusion coefficient increase the B term, while increased viscosity decreases it